The stability of confined radio jets - The role of reflection modes

The linear stability of a confined radio jet is reinvestigated. The roles of both absolute (temporal) and convected (spatial) instability are considered, and it is demonstrated that the two are related through the group velocity. The dispersion relation is analyzed asymptotically for the fundamental and reflection modes. Numerical results are presented for pinching modes. A geometrical interpretation of the modes is presented in terms of the propagation angle and is visualized by contour plots of the pressure perturbation. A confined jet theta M = arc sine (1 + sq rt eta)/M, where eta is the ratio of the internal to the external gas density and M is the Mach number of the jet. The connection between these linear modes and the cross-shaped shock patterns which are seen in laboratory jets, and the implications of these calculations for the development of large-scale features in extragalactic radio jets, are discussed.